This code writes the string to a file. It is important to close the writer, so this is done in a finally block.

  public void writeLineToFile(String str) throws IOException {
    File file = new File("file.txt");
    BufferedWriter bw = null;
    try {
      bw = new BufferedWriter(new FileWriter(file));
      bw.write(str);
    } finally {
      if (bw != null) {
        bw.close();
      }
    }
  }

Also note that write(String s) does not place newline character after string has been written. To put it use newLine() method.

public void writeLineToFile(String str) throws IOException {
    Path path = Paths.get("file.txt");
    try (BufferedWriter bw = Files.newBufferedWriter(path)) {
        bw.write(str);
    }
}